General-Purpose Mail Filter

1.2 Overview of Mailfromd

In contrast to the most existing milter filters,
mailfromd does not implement any default filtering
policies. Instead, it depends entirely on a filter script,
supplied to it by the administrator. The script, written in a
specialized and simple to use language, called MFL
(see MFL), is supposed to run a set of tests and to decide
whether the message should be accepted by the MTA or not.
To perform the tests, the script can examine the values of
Sendmail macros, use an extensive set of built-in
and library functions, and invoke user-defined functions.